The influence of the place-value structure of the Arabic number ...

Sources of difficulty in carry addition problems
On a theoretical/conceptual level difficulties arising with the requirement of at least
one carry have been suggested to originate from different sources. On the one hand, Kalaman
and LeFevre (2007) argued that the carry effect represents an increased demand on working
memory resources for e.g., keeping track of intermediate results (see also Imbo et al., 2007;
Kazui, Kitagaki, & Mori, 2000; Zago et al., 2001). On the other hand, Nuerk, Graf and
Willmes (2006) associated the increased difficulty of carry addition problems with a higher
workload for the correct classification (i.e., units, tens, etc.) and manipulation of digits within
the base-10 structure of the Arabic number system (i.e., carrying from one position to
another). Finally, Green, Lemaire, and Dufau (2007) suggested that the carry effect may also
reflect processes necessary to adapt solution strategies to the actual problem (see also
Torbeyns, Verschavel, & Ghesquiere, 2002). In sum, these data suggest that there seem to be
different sources of difficulty for the carry operation.
At this point, evaluating eye fixation behaviour may supplement the interpretation of
response latencies and error rates when investigating the temporal dynamics of number
processing (cf. Brysbaert, 1995). According to the immediacy and eye mind assumption (i)
the eyes tend to fixate those objects from which visual information is extracted to support
their cognitive evaluation (e.g., Just & Carpenter, 1980; Rayner & Pollatsek, 1989).
Additionally, (ii) fixation durations are agreed to reflect a reliable measure of how long
processing of a particular stimulus takes (for a review see Rayner, 1998). Thereby, evaluation
of participants’ eye fixation behaviour may be a valuable tool to dissociate cognitive
processes in numerical cognition.
However, in contrast to other neuro-cognitive domains such as e.g., reading research
only few studies in the domain of numerical cognition have yet tapped the potential of eyetracking
data. Nevertheless, the eye-tracking methodology has already been employed to
study basic as well as more complex numerical processes. First evidence for the validity of
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